Method for fractional distillation



May 15, 1951 J. A. PELLETTERE 2,553,469

METHOD FOR FRACTIONAL l-DISTIIJATIQI Filednec. ze, 194s J O SEPi- P. PEI-'LET T/ERE Patented May 15, 1 951 METHOD Foa FRAcTloNAL Dls'rmLA'rIoN Joseph A. Pellettere, Pittsburgh, Pa., -assignor to Gulf Oil Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Application December 26, 1946, VSerial No. 718,279

2 Claims.

This invention relates to a method for fractional distillation and, more particularly, it is concerned With maintaining a constant reflux ratio in fractional distillation.

In the ordinary practice of fractionating petroleum distillates, unavoidable fluctuations or changes in load in various parts of the apparatus result in uneconomical and inefficient operations. In order to avoid such fluctuations as much as possible, the prior art has recognized that certain factors in the distillation operation should be maintained as constant as possible. Thus, it is common practice to attempt to provide a constant rate of feed of the mixture to be fractionally distilled, a constant pressure on the fractionating column, a constant heat input, and a constant reux ratio in the column. Of these factors, that of providing a constant reflux ratio has proved the most difficult to carry out satisfactorily.

In the prior art, various methods and means have been employed to provide a constant reiiux ratio, and assuming that the rate of feed, heat input and the pressure on the fractionating column are maintained constant, it has been attempted to maintain a constant level of condensate in the reiiux accumulator, thereby to permit a constant rate of flow of reflux to the column. By maintaining the level of the condensate in the reux accumulator substantially constant, and with the other factors above mentioned being constant, it is possible to maintain a constant reflux ratio throughout the distilling operation. However, the prior art attempts have not been particularly successful.

In one prior method for maintaining a constant level in the reux accumulator, it has been attempted automatically to control the rate of condensation in accordance with the level of condensate in the reflux accumulator, the rate of condensation being increased when the level falls and, conversely, the rate of condensation being decreased when the level rises. This method of solving the problem has not proved satisfactory in practice. When the level of condensate begins to fall, more Water is supplied to the condenser to increase the rate of condensation, but by the time additional condensate is obtained, the accumulator is empty and no condensate is available to serve as liquid reflux. At this time, the rate of condensation becomes so high that the accumulator fills to the top before the condenser responds to the increased level and reduces the flow of condensate. The net result is a cyclic operation with the accumulator alternately empty and brim full. Obviously, under such circumstances a constant reflux ratio cannot be maintained. Other means of control have been attempted, but have not been successful.

It is an object of this invention, therefore, to

provide a method of fractional distillation Where-v ating zonev is condensed at a rate to produce an excess of condensate over that required to pro--v duce liquid reflux for the fractionating zone, and

the excess of condensate so produced is vaporized Y by supplying heat to the condensate, thereby maintaining a constant level of condensate in the reux accumulator and permitting a constant rate of flow of reflux liquid to the fractionating zone. ,1

More particularly, I have found that in fractional distillation processes, particularly where it is attemptedv to separate light hydrocarbon fractions suchas C3 and C4 hydrocarbon mixtures, if an excess of condensate is produced over 1 that normally required to supply liquid reiiux to the fractionatingvcolumn, and if such excess is vaporized by supplying heat to the condensate,

it is possible to maintain a constant'level in thev reflux accumulator and to obtain a constant reflux ratio in the fractionating column. In producing the excess of condensate, the condenser is so set that the rate of flow of cooling medium, such as water, through the condenser is suicient to condense the maximum'possible amount of vapors that may pass through the condenser by reason of variations in load atY any time. In accordance with my invention, the reflux accumulator is provided with means to vaporize such excess by supplying heat thereto, thereby avoiding the accumulation of more condensate than can be used for reflux, and maintaining a constant level of reflux in the reflux accumulator.`

A convenient method of accomplishing such vaporization is to pass a portion of the Aconden-4- sate through a tubular heat exchanger inwindirect heat exchange relationship with a heating medium, such as steam. kThe excess condensate so vaporized may then bedisposedof in anydesired manner. In accordance with my invention, if the level of condensate in the reflux accumulator tends to rise and thus produce an excess of condensate over that required for reflux, that excess is substantially immediately 5 vaporized. Conversely, if the level of condensate in the reflux accumulator tends to fall below the optimum requiredrfon-reflux, less vaporizationonV no vaporization of condensate takes place and the level of condensate will tend to rise to ther optimum point.

My invention will be fully@ understoorl fromf. the following description taken with reference to the attached diagrammatic drawings, iniwhich. Figure l represents a preferred'form of lan ap- 1 paratus for practicing the invention, and Figure 2 represents a modification of` the; vaporizing: i means of Figure 1.

Referring to Figure 1 there is shown a fractionating column lr6 providediwith safety valves Hiandu, a reboilerl! 'and feedlines I3, Wand I 5": Thefractionating Vcolumnw is' connecte d with aA condenser 28,' refluxacc'umulator 3Q provided with a safety' valve-"31, and reflux pump 33 throught' suitable connections, as-is" more fully set forthbelow. Adjacent' tothe reilux accumulator' there" isprovided a-conventional tubular heat exchanger, such as the hairpinV exchanger- In operation;l whichwill bedescribed with referenceV tO-theeparationof a mixture of C3 and C4- hydrocarbons, the` mixed liquid feed is passed through-pump-HL conventional recording flowffcontrollerl l., line.` |2and multiple feed lines |32, 14 and. I 5=to suitable plates in Vthefractionatingr column: I6; The recording flow controller H'maintainsaconstant ratelof ,flow of the feed. Iromlthe` fractionati-ngcolumn IS, Ca-h'ydro carbons: aretaken Loverhead through lineand Ch hydrocarbons: are' takenas. liquid bottoms through.` line 22',cooler' 23; and line 24-and are passedito storageor-any other desireddisposition.

The-conventional level controller l 9 .in-the base of: the; fractionatingacolumnv iB serves. automaticallyrtmaintain the .liquid .levelint the column. 4f Heat for the operation of the column is provided byreboilerl', .using steamior. debutanized bottomsifromaaprevions fractionation as: the heating medium., rl'he rehoiler;2i.;is;provided with a conventonalrecgrdng 'ljCvW, controllervr 2|v to main- 50 tain.v the;l llowof; heating; medium i substantially constant and. thus,`- supply; heat; at au constant. rateitovthe mixture to fbeffractionated., Asshown iryntheAA drawing,. .the controLvalve; 21a used A with ow. controller 2;.l i iszplaced; in lzthe bypass Vof Y the. heatingmedium line.Y However, .the control valve 21a` may be placed 1 in the, heating; mediumline either. in theentrancegto orexit from the reboiler particularly when steam.y is` used-as theheating medium. Asfshown in the drawing, the reboiler operates. on the thermo-syphon principle as is welliunderstoodinthe art. The overhead vapor-izedtop product, consisting, of C3 hydrocarbon vapors, passes.. through conventional recording pressurecontroller.2-6,.which.maintains the pres.-

sure on the, column .constant under i all conditions ofloperation, and then passes through line. 21, condenser 2.8'. and line v29 into refluxaccumulator 3l). Therate of Yflow. of cooling medium, such asfwatergthrough condenser 28 is setby hand 70 so as-slightly'to overcondense,that is; to condense z the maximumlpgossible amount `of C3 hydrocarbonv vapors which-may'berproducedzloyJ reason of load iuctuationsev or f changes;V the'. reflux: accumulater-l ,thelevellofccondensateaccumulatedftherein' 4 is maintained substantially constant as follows. A portion of the condensate flows through line 36 into the shell of the hairpin heat exchanger 31, supplied with steam or other suitable heating medium in the tubes through lines 38 and 39. The portion of the condensate entering the heat exchanger is vaporized and passes through line 4B; conventional recording flowl meter f 42, conventional back pressure regulator 43 and line Mto any desired disposition, such as fuel. Any surplus uncondensed vapors in the reilux accumulator:y 3.0, flow" through line 4I to join the vaporized excess condensate in line 44. Liquid C3 rhydrocarbon refluxAv` passes fromY accumulator 5 3ll"throughline 32`;`,reflux pump 33, conventional recording"ow'controller 34 and line 35 to the topfofrthefractionating column I6 at a constant rate. The level of condensate in the reflux accumulator is maintained approximately in the center of the heat exclianger 31. If the level of condensatel begins torise? more= condensate enters theeh'eatf exchanger; more-ofi thei heated? surface containedJthereinlisacoveredf by the-con'4 densate and, therefore; moreheat' is@ suppliedl to .the condensate causing a'. faster rate of Vaporizationfan'df bring-ing-TV thef-leVel-l of the icondensate: down:` If the levell of the'- condensatev in.. the accumulator-begins-to fall lessloff'the condensate enters heat vexchanger:31 andf-lessof the heating v surfacegtherein islexpose'dlto-the liquid, therebyv decreasing: the amountof vaporization ofi the condensate.andincreasingthe level of? condensate*4 accumulator. Accordinglyaby thisfrneans Iv avoid; any.l` uncontrolled .surges of condensate; thes'greilux pump; operateslunder al substantially.: constant; head andi inicorn'bination. with irecordV ing flowA controllers'34;i.furnislies1a; constantlrate` ofi flow# of: reflux; to: they fractionatingi columm, therebyassuringf atl-constant; reux ratio..

In the modication shown in Figure-2;l the-f hea-tf exchanger :3 1 may,x bef. supportedton supports 45.- and 4B.f,o1r.theiground;, thuspermitting easier servicing.u The mannenozoperation-offthis modi@ cation: isa similar t that'. shown in- FigureA 1'. Any condensatei exceeding the: optimumr level,.- shownv as dottedilineclllis Avap'orizeds-inf the heat'` exchanger andiifitheslevelsfallszbelowf'thisV point,` no. Vaporiza-tion takesxplacec.

While-,Iihaveidescribed myinvention withpan-f ticular reference:rv to; separationvv of.' at mixturev of' Carand C4,hydrocarbons;.itissnot limited 'thereto butmayffbe; usedi for; any fractional distillation, particularly where?. the.l overhead; product of such"- distillation lisxeasilygvapnrized. Althoughfl have:v shown a tubular hairpin heatexchan'gerfasvthe vaporizing; means;.othen meansiofi supplyingY heat to. -the condensate accordance-2 withi the prin-- cpleof. my invention/may loe-2` employed,A asLw-ill be Y' apparent toithoseskillediin;tlrei art.

I claimtA 1. In; a; continuous; processi ofi fractionalk dise tillation; whereinazmixturesoficomponentsisfsep-` aratedfiirto; liquid bottom kproduct and.V vapor-ized topfproducttin'; aw fra'cti(matingi zonef and wherein `thexvaperize'd:A top;2 productfV isi condensed` and aportionzther'eof is returned to-theffractionatingf zone a'fconstantiratekto serve-fasiliquid reux therefor; the' method; offmaint'ainin-ge a-constant" reflux.ratioinisaidfiactinating'zone which com- .prises,z maintaining a2A constant?" rate ofA feed 'to-'v the",` fractionatiiig zonegmaintaining a constantl ratei of l'eatf' input tq'-V theL fractionating,I zone; maintaining aconstantl'pressureon the fractionating' Zone',Jv condensing the`4vv vaporized top` productat a ra-t`e to'produce"an excess offcon densate over that required to produce liquid reflux for the fractionating zone, accumulating a pool of condensate, and maintaining the level of said pool of condensate substantially constant by removing said excess of condensate from said pool of condensate by Varying the heat supplied to said removed excess of condensate in accordance with the Variation in level of said pool of condensate to vaporize the above-mentioned excess of condensate.

2. A process for the separation of a mixture of Cs and C4 hydrocarbons by fractional distillation which comprises supplying said mixture to a fractionating zone at a constant rate, supplying heat to said mixture in the fractionating Zone at a constant rate, maintaining a constant pressure on the fractionating zone, withdrawing liquid C4 hydrocarbons from the bottom of the fractionating zone, withdrawing vaporized C3 hydrocarbons from the top of the fractionating zone, condensing said Voporized C3 hydrocarbons at a rate to produce an excess of condensate over that required to produce liquid reux for the fractionating zone, accumulating a pool of liquid C3 hydro- REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,563,097 Milker Nov. 24, 1925 1,963,841 Frankl June 19, 1934 2,160,898 Peff June 6, 1939 2,348,659 Smith et al May 9, 1944 2,355,589 Brandt Aug. 8, 1944 2,423,156 Reid July 1, 1947 2456,398 Gerhold Dec. 14, 1948 

